Ten replications of individually penned gilts from a high-lean strain were used to determine the effect of ractopamine (RAC) on the optimum dietary available phosphorus (AP) regimen. At 70 kg body weight, pigs were randomly allotted to a corn-soybean meal basal diet (.08% AP) adequate in all nutrients except AP. The basal diet was supplemented with mono-dicalcium phosphate to create six AP concentrations (.08, .13, .18, .23, .28, .33%) and ractopamine HCl to create two RAC concentrations (0 and 20 ppm). A constant Ca/AP ratio of 2.5:1 was maintained in each diet. BW gain and feed intake were recorded weekly for 5 weeks, and total urine output was collected via urinary catheter the last 2 days of each 7-day period. After this 5-week collection period pigs remained on their diets until they achieved a body weight of 114 kg and were then slaughtered keeping the ham and loin for subsequent dissection and bone removal.

In pigs growing from 70 to 114 kg BW dietary AP additions resulted in improved (P<.01) daily BW gain, but did not alter carcass or ham-loin muscle content. Dietary AP additions also linearly improved (P<.01) bone integrity as observed by ham-loin bone content and femur and fifth vertebra weight and mineral content. RAC improved (P<.01) BW gain (+125 g), gain/feed ratio (+64 g/kg), and carcass and ham-loin muscle (+3.4%, +5.6% ) content. RAC reduced (P<.05) the ham-loin bone content and femur and fifth vertebra weight and mineral content, but the amount of additional bone or bone mineral accrued per unit of added dietary AP was linear and independent of RAC. Because of their greater muscle accretion capacity, thus P demand, pigs fed RAC from 70 to 114 kg BW needed an additional .03% AP to maintain the same femur mineral content as the non-RAC pig.

Over the five periods, RAC lowered P content of BW gain (4.66 vs. 4.05 g/kg) and urinary P excretion (219 vs. 67 mg/d) independent of dietary AP. The magnitude of change in BW gain and P content of BW gain was reduced in later periods of growth. Dietary AP additions improved (P<.01) P accretion and P content of BW gain. To achieve maximum BW gain, the amount of AP needed was not altered by RAC. To achieve the same P content of BW gain, an indicator of the adequacy of the bone mineral content of gain, an additional .06% to .02% AP was needed in the RAC pigs depending on length of ractopamine feeding. Based on these data, RAC does not alter the amount of AP needed to optimize rate and efficiency of BW gain but does increase the AP needed to maintain P content of body growth equivalent to non-RAC pigs.